1. Field of the Invention
The present invention relates to an aberration measurement apparatus and an aberration measurement method. More particularly, it relates to an apparatus and method for measuring the aberration of an imaging optical system for use in a laser process.
2. Description of the Related Art
In various laser processes, light passed through a light modulation device (such as a phase shifter, a photomask or a reticle) is generally imaged on an irradiation surface by an imaging optical system. Here, the laser process is “substance processing by laser” in a broad sense, and is a process which uses characteristics inherent in laser light, that is, directivity, monochromaticity and light collecting properties (high energy density) to produce various physical and chemical changes in the irradiation surface and/or a material having an irradiation surface.
As one example of laser process technology, there have been known a projection type crystallization device and method wherein excimer laser light is applied to a light modulation device such as a phase shifter, and light having a predetermined light intensity distribution which has passed through the light modulation device and which has been formed by an imaging optical system is applied to a nonmonocrystalline semiconductor film (polycrystalline semiconductor film or amorphous semiconductor film) to melt this film so that a crystallized semiconductor film is formed. In the above device and method, a temperature gradient is produced in the melted region on the nonmonocrystalline semiconductor film in accordance with the light intensity distribution, and a crystal nucleus is formed in conformity to a point with the lowest light intensity, and then a crystal grows from the crystal nucleus toward its periphery such that a large-diameter crystal grain is created.
In a laser process device, a high-output light source such as an excimer laser, YAG laser or CO2 laser is generally used. In this kind of device, as a light flux having high energy density passes through the imaging optical system, optical components such as lenses and mirrors constituting the imaging optical system and associated components such as lens frames and a housing (the optical components and the associated components are hereinafter generically referred to as optical members) are heated by light irradiation, and thus deform due to thermal expansion. In order to prevent the deformation of the optical members due to the thermal expansion, the imaging optical system is cooled off using, for example, a radiator plate, a liquid (water), a gas (air) or a Peltier element.
Good imaging performance, that is, reduced aberration is required for the imaging optical system used in the laser process device. Thus, various methods of measuring the aberration of the imaging optical system have been developed and carried out. Typical known aberration measurement methods of the imaging optical system include, for example, a measurement method using various interferometers, Hartmann method, Shack-Hartmann method, Foucault method and Ronchi method.
As described above, in the laser process device, the optical members constituting the imaging optical system are heated by the light flux having high energy density and thus deform due to thermal expansion. When the optical members deform due to the thermal expansion in response to the light irradiation in this manner, there is a possibility that the aberrant state of the imaging optical system deteriorates with time. Therefore, there has been a desire in connection with the laser process device to measure in real time the change of aberration when the light flux having high energy density is applied to the imaging optical system in a state close to an actual use condition.
However, in a conventional aberration measurement apparatus and method, the aberration of the imaging optical system is measured in a condition completely different from the actual use condition in the laser process device. Specifically, a light modulation device corresponding to a pattern to be processed is provided on the object plane of the imaging optical system in the actual laser process, whereas a pin hole is provided in the object plane of the imaging optical system in, for example, the Shack-Hartmann method and shearing interferometry. As a result, in the prior arts, weak light which has passed through the pin hole only passes through the imaging optical system, and it is therefore impossible to measure the change of aberration with time in the light entrance state close to the actual use condition.